Fibrous, nonwoven, superabsorbent, polymer-impregnated structures are known. See generally, U.S. Pat. Nos. 5,167,764, 5,607,550, 5,516,585. European Publication No. 437,816 discloses a wet laid process for incorporating superabsorbent which entails forming a gel on mixing of the SAP particles and fibers in a slurry. The amount of SAP contained in the webs taught is up to 60% of the total weight of the web. The particular superabsorbent particles used in the process taught in EP 437,816 yield a web which exhibits a characteristic absorbency under load (AUL). Higher AUL has been achieved for SAP more recently, however the use of these higher AUL SAPS in a continuous large-scale wet lay process presents serious difficulties. Attempting to use a commercial wet lay process in light of the teachings of the state of the art will present serious problems for example if fine particle size SAP (100 micron or less) or SAP particles which are surface crosslinked are attempted. The small particle size (less than 200 micron ) SAP or surface crosslinked SAP or particle range 200 to 850 micron can form a gel and result in a non-uniform web and a web which cannot be dried by practical means.
Alternatives to the gellation problem include EP-A-359615 which discloses a method for the manufacture of a superabsorbent fibrous structure in which a dry solid absorbent is applied directly to a wet-laid web of cellulosic fibers prior to drying the wet web.
EP-A-273075 discloses a high water-absorbency paper made by sheeting a mixture of wood pulp fiber, water-soluble resin and high water-absorbency resin.
Absorbent products such as diapers which include particles of a superabsorbent polymer such as crosslinked sodium polyacrlate disposed between layers of wood pulp are known for example from EP-A-257951.
The use of fibers of water-swellable water-insoluble superabsorbent, polymer is disclosed in U.S. Pat. No. 5,607,550, wherein it is taught that incorporation of superabsorbent, polymers in particulate form in the fiber web have significant disadvantages in many respects. The prior art teaches that superabsorbent, polymer particles are less securely retained and with less uniform dispersion of superabsorbent particles as opposed to the dispersion of the fibers of SAP. It is also taught conventionally that with superabsorbent, polymer particle-impregnated structures, the particles are loosely attached to the fibrous structure of the nonwoven fabric and attrition or loss is evident.
In order to provide sufficient absorbency performance for utilization in state-of-the-art absorbent articles, it has been found that a minimum 0.3 psi AUL of 30 for the SAP is needed and desiredly the percent loading of superabsorbent in a fibrous web needs to be at least about 50% by weight. However, loadings of SAP particles in a fiber structure (such as above about 80% SAP particles on the total weight of the web) have insufficient strength for the wet web to convey through the wet-lay forming process.
Whereas the cost associated with forming fibers of superabsorbent, polymer is inherently higher than that of the particulate SAP, it would be desirable to overcome the aforementioned drawbacks in the use of particles of SAP. Composite structures of fibers impregnated with superabsorbent, polymer particles could greatly reduce the manufacturing cost of end use products such as those aforementioned.
Co-pending U.S. Patent application Ser. Nos. 09/026,002 and 09/025,384 disclose process to make SAP/fiber composites by the wet laid nonwoven method and utilize added salt in the furnish to retard the gellation of the SAP. The presence of salt gives rise to inefficiencies in the process as well as environmental compliance issues. Therefore it would be desirable to eliminate salt addition to the furnish. Accordingly, there is a need for an environmentally friendly process to make SAP/fiber composite on commercial scale wet-lay equipment and at sufficient line speeds to be of commercial economical importance. An improved process for forming such a composite has been found which yields a uniform web which can be dried using conventional drying equipment. The web also exhibits advanced absorbency performance.